Due to present environmental concerns over the use of potentially ozone-depleting or flammable blowing agents, it is desirable to make styrenic polymer foam structures with a blowing agent containing carbon dioxide. Such foam structures blown with carbon dioxide can be seen in U.S. Pat. Nos. 4,455,272, 4,559,367, and European Patent Application 89114160.8.
A problem with using carbon dioxide as a blowing agent for styrenic polymer foam structures is its relatively low solubility in melts of the styrenic polymer. The low solubility results in high system pressure, which manifests itself in high pressure drop and excessive die pressure. Lowering pressure drop and die pressure would save energy costs, improve skin quality, and reduce the pressure duty requirements for process equipment. Low carbon dioxide solubility also results in small cell size and high density. Increasing carbon dioxide solubility would increase cell size and lower density, which in turn permits larger potential foam cross-section.